1. Field of the Invention
The invention relates to an improved microwave moisture indicator and control device and more particularly to a microwave moisture indicator which directs microwaves obliquely toward a web of material and detects microwaves reflected from the web of material, the detected signal being proportional to the moisture content of the web of material.
2. Description of the Prior Art
Microwave moisture measuring devices are broadly old in the art as indicated by the following patents known to applicants:
U.s. pat. No. 3,534,260 PA1 U.s. pat. No. 3,639,834 PA1 U.s. pat. No. 3,913,012 PA1 Gt. Britain Pat. No. 1,052,250
The present invention utilizes the principle that microwave energy reflected from a sheet or web of material is highly dependent upon the moisture content of the material. Thus, according to this invention modulated microwave signals are directed toward a web at a selected optimum angle of incidence, and only that part of the microwave signal which is reflected by the web is detected and measured.
The closest prior art device knwon to applicants which utilizes the principle identified in the preceding paragraph and thus which measures only that part of the microwave signal which is reflected by the web is British Pat. No. 1,052,250.
U.S. Pat. No. 3,534,260 and 3,639,834 each disclose microwave moisture measuring systems which direct microwaves through a sheet of material, such as paper, reflect the microwaves back through the sheet and detect the microwaves on the same side of the sheet from which the microwaves were transmitted. A portion of the microwave energy is absorbed each time it passes through the web. The measurements made by the apparatus disclosed in U.S. Pat. Nos. 3,534,260 and 3,639,834 is a function of the attentuation of the microwave energy transmitted through the web. As recognized by British Pat. No. 1,052,250, a disadvantage of the process representedby U.S. Pat. Nos. 3,534,260 and 3,639,834 is that the web moisture content is only very slightly dependent upon the absorbed microwave energy, thus large differences in moisture give only slight differences in absorption, so that measurement is inaccurate.
U.S. Pat. No. 3,913,012 discloses a microwave moisture measuring system whose measurement is dependent upon the microwave absorbtivity of a workpiece. The system disclosed in U.S. Pat. No. 3,913,012 is subject to the same disadvantages as recognized in British Pat. No. 1,052,250.
The present invention is an improvement over the method and apparatus disclosed in British Pat. No. 1,052,250 in that the present invention produces a linear or straight line relationship between the moisture in the web and the reading of the measuring instrument whereas the method and apparatus disclosed in the aforesaid British patent produces a non-linear relationship as shown in FIG. 2 of the British patent.